1. Field of the Invention
The present invention relates to a velocity detection method for detecting a relative velocity between a recording medium having a plurality of tracks and a conversion means for recording/reproducing the recording medium, and to an apparatus effecting such method.
2. Description of the Prior Art
In an optical type recording and reproducing apparatus, a recording medium (hereinafter referred to as "disk"), obtained by forming a thin film of an optically recordable and reproducible material, through a process such as vacuum evaporation, on a surface of a substrate having uneven-structured concentric tracks, is irradiated with a light beam generated by a light source such as a semiconductor laser diode and focused by a condenser lens. In signal reproduction, a signal is read from reflection light from the disk under the condition that the light beam is made to have a relatively weak fixed light amount, while in signal recording, a signal is written on the disk under the condition that the light amount of the light beam is modulated to be weak and strong in accordance with the signal to be recorded.
Such an optical type recording and reproducing apparatus performs are focusing control for controlling the light beam so that the light beam is always substantially in a predetermined focusedcondition on the film of recording material and tracking control for controlling the light beam so that the light beam is always positioned on a track. Further, in order to enable the light beam to make random access or search on tracks on a disk, track search control is performed in which the light beam is moved toward a target track in the radial direction of disk while the tracking control is made inoperative, and the tracking control is made to operate again upon arrival of the light beam at the target track. As for the prior art with respect to track access or search operation, reference is made to U.S. Pat. Nos. 4,106,058 and 4,332,022.
One of the important matters for performing track access or search is the speed of displacement of the light beam when the light beam is caused to enter the target track, that is, the tracking pull-in speed.
The frequency bandwidth of tracking control is finite and the limit thereof is generally about several KHz. Accordingly, if the tracking pull-in speed is too high, a failure may be caused in pull-in of the tracking control into the target track. If the tracking pull-in speed is too low, on the contrary, the time required for the track search operation is prolonged.
When the light beam is displaced in the radial direction of the disk in the track search operation, accordingly, velocity control for controlling the velocity of the light beam is performed so as to control the tracking pull-in speed with accuracy to perform stable pull-in of the tracking control into the target track.
The track search operation is performed by displacing the light beam in the radial direction of the disk so that the velocity of movement of the light beam is in accord with a predetermined reference velocity corresponding to a current position of the light beam in the track search operation.
The current velocity of the light beam necessary for performing the velocity control is detected from the period of a track accessing signal which is caused when the light beam runs across tracks, as will be described later in detail. The current position of the light beam can be obtained by counting the track accessing signal caused at each of the tracks from the start track from which the track search operation is started. A tracking error signal and a track accessing signal are shown in FIG. 2. In FIG. 3, the diagram (a) shows the state in which the light beam runs across tracks on a disk and the diagram (b) shows a tracking error signal at that time.
JP-A-59-9085 (Feb. 29, 1984) and French Patent No. 7529707 (Sep. 29, 1975) teach in detail that the tracking error signal may be extracted by a push-pull method as shown in the diagrams (a) and (b) of FIG. 2 under the condition that the wave length of the light beam is .lambda. and the optical depth of the unevenstructured tracks is about .lambda./8. Accordingly, explanation of how to extract the tracking error signal is omitted here.
In FIG. 2, the diagram (c) shows a waveform of a signal obtained by binary-digitizing the tracking error signal and the diagram (a) shows an edge detection signal obtained by detecting the leading edges of the waveform of the diagram (c). This edge detection signal is caused when the light beam runs across the center of each track as seen from the diagrams (a) and (d), and the edge detection signal is therefore a track accessing signal. Accordingly, the count value obtained by counting the edge detection signal, that is, the track accessing signal, from the start of the track search operation represents the current position of the light beam. Further, since the tracks are provided on a disk at substantially regular intervals P in the radial direction of the disk, the velocity V of the light beam is expressed by the following equation: EQU V=P/T
where T represents the period of the track accessing signal.
In FIG. 2, the diagram (e) shows a signal obtained by detecting the leading and trailing edges of the waveform of the diagram (c). If the period of the signal of the diagram (e) is represented by t, the velocity V of the light beam is expressed by the following equation: EQU V=P/2t.
However, fine dust or streaks may exist in a disk so that there occurs dropout due to such dust or streaks. Further, device noise may exist in the apparatus. If such dropout or device noise is mixed into a tracking error signal, noise in the form of pulses occurs in the binary-digitized signal so that the period of the track accessing signal becomes extremely short. Since the velocity of the light beam is detected on the basis of the period of the track accessing signal, the velocity of the light beam may be erroneously detected so as to be an extremely high speed. As a result, there has been a problem in that a command signal for reducing the velocity of the light beam may be erroneously issued so as to generate abnormal velocity fluctuations to thereby cause a failure in pull-in for the tracking control.